rs6000: Some builtins require IBM-128 long double format (PR103623)

rs6000: Some builtins require IBM-128 long double format (PR103623)

2021-12-14  Bill Schmidt  <wschmidt@linux.ibm.com>

gcc/
	PR target/103623
	* config/rs6000/rs6000-builtin-new.def (__builtin_pack_longdouble): Add
	ibmld attribute.
	(__builtin_unpack_longdouble): Likewise.
	* config/rs6000/rs6000-call.c (rs6000_expand_new_builtin): Add special
	handling for ibmld attribute.
	* config/rs6000/rs6000-gen-builtins.c (attrinfo): Add isibmld.
	(parse_bif_attrs): Handle ibmld.
	(write_decls): Likewise.
	(write_bif_static_init): Likewise.